Cycles compositor - apply color balance node only on a part of image
Based on the information in your question I am assuming you already created the mask.

Instead of combing the two results after you have colored corrected the one you could restrict the color correction to a specific region.

To do this feed the mask into the Fac Input of the Color Balance Node.



You can also use an image as a mask. Plug the image into the Fac Input of the Color Balance Node. Note that white means apply, black mean ignore. THis mean that the color balance will only be applied where the image is white. In my case I had to invert the colors to only apply the color balance on the scribble.